# Ideal gas or not ideal gas

1. Dec 8, 2009

### sportsstar469

what are the differences? my professor said something like an ideal gas has a big temperature, and a big volume. and ithink he says it follows the kinetic molecular theory. but i also know that different gases react at different temps and volumes so big is relative. how do i know what is big for which gas?

also he said its ideal if the gas matches what the ideal gas equation is saying (PV=nRT)
i dont understand what he means by that. i know how to manipulate the equation but i dont know what he means by that. im pretty sure on thursdays quiz, im gonna have to explain this.

its probably a simple concept, but im just stressed anyway, i dont want to lose my in this class, by failing the final

2. Dec 8, 2009

### mgb_phys

An ideal gas is an approximation of the real world to make the maths simpler.
If you look at PV=nRT is says that at T=0 the gas has no volume and no pressure, so where have the atoms gone? Thats because we assume the 'ideal' gas molecules have no size.
We also assume there is no interaction between molecules of the gas - so they don't bounce off each other or attract each other.

At large volumes or low pressures gases become more ideal. The size of the molecules becomes negligble compared to the container.
At very small volumes and very high pressures - imagine a lot of gas compressed into a very tiny space - you can't ignore the size of the molecules and it isn't ideal

3. Dec 8, 2009

### sportsstar469

im assuming since volume is directly related to moles, and temperature, that ideal gases have high volume, high moles, and high temps, but low pressures?

4. Dec 10, 2009

### Thermodave

The ideal gas law is based on seveal assumptions. First, the gas is made up of many, many molecules that move randomly. This is necessary in order for the molecules to collide, exchange energy, and reach equilibrium. The model also assumes that the actual volume of the particles themselves is very small compared to the total volume the system takes up. This is why it says that there is no volume at T=0, since at that temperature the volume would just be that of the atoms (no free space between). Finally, the law assumes no forces are acting on the particles except for when they collide. These assumptions work out pretty well for a lot of gases and the law has been known for centuries and is found by simple kinetic theory using these assumptions. You can also see the Van der Waals gas law which accounts for atomic forces and volume.

5. Dec 11, 2009

### mgb_phys

Ideal gas laws assume there are no forces between the molecules and only collide with the walls of the container not with each other. At high pressure, as you squeeze more molecules into a space, the number of collisions between molecules increases